"insufficient memory" Reisntall may have reset/changed the amount of memory that SQL can acquire

"disk space" Check you are not approaching Disk Full on any drives that SQL might be wanting to use.

"I tried copying an older version of mssqlsystemresource.mdf & ldf to no avail"

That doesn't sound good - unless the COPY you made was with the SQL SERVICE stopped. Do you have a Backup file? that's much more likely to be usable than a file-copy (unless SQL Service was stopped when you made the COPY; if it was then go through the process of ATTACHing the files, and NOT just copying them back into their original location.)

Hopefully Gail will pop-up here soon, then you'll get some proper advice